Field of the Invention
The present disclosure relates to color misregistration correction control of an image formed by an image forming apparatus.
Description of the Related Art
The image forming apparatus forms a color image by, for example, forming toner images of different colors on a plurality of photoreceptors and overlappingly transferring the toner images on a recording medium such as a sheet. The image forming apparatus may directly transfer the toner image to the recording medium from the plurality of photoreceptors. Alternatively, the image forming apparatus may transfer the toner image to an intermediate transfer member from the photoreceptor. Thereafter, the image forming apparatus may secondarily transfer the toner image to the recording medium from the intermediate transfer member.
The image forming apparatus as mentioned is configured such that the toner images formed on each of the plurality of photoreceptors accurately overlap each other. However, due to component tolerance of the image forming apparatus and/or influence of a change in position of the component caused by a temperature change when forming the image and the like, a situation in which the toner image may not overlap each other on the recording medium, i.e., color misregistration, may occur. Thereby, the image forming apparatus performs control for correcting the color misregistration.
To perform the color misregistration correction control, for example, the image forming apparatus forms a color misregistration pattern (measurement image) for each color on an image carrier such as the intermediate transfer member. Then, the image forming apparatus measures a relative position of the image pattern of each color. The image forming apparatus calculates an amount of color misregistration based on the relative position of the image pattern of each color. Then, to reduce the amount of color misregistration, the image forming apparatus corrects an image forming position of the toner image of each color.
The color misregistration pattern is measured by an optical sensor. The optical sensor has a light emitting part and a light receiving part. When measuring the color misregistration pattern formed on the intermediate transfer member, the optical sensor irradiates light to a region where the image pattern is not formed and to the image pattern of the intermediate transfer member from the light emitting part. Then, the optical sensor receives its reflected light through the light receiving part. The light receiving part outputs analog signals in accordance with light intensity of the reflected light (a reflected light amount) to be received. Thereby, the light receiving part outputs the different analog signals in accordance with the reflected light from the intermediate transfer member and the reflected light amount from the image pattern. The image forming apparatus compares a value of the analog signal which is output from the light receiving part with a threshold to convert the analog signal into a digital signal (binary). Based on the digital signal, the image forming apparatus detects the relative position of the color misregistration pattern of each color on the intermediate transfer member.
The intermediate transfer member wears when the toner image is transferred to the recording medium. The intermediate transfer member also wears when the toner on the intermediate transfer member is removed by a cleaning unit. Further, toner and paper dust may adhere to a surface of the intermediate transfer member. When the intermediate transfer member wears or the toner and the paper dust adhere on the surface of the intermediate transfer member, the optical sensor may find it impossible to accurately detect the relative position of the image pattern. In particular, surface reflectance of the intermediate transfer member is reduced so that the reflected light amount is reduced. Due to this, a difference between the reflected light amount by the image pattern and the reflected light amount of the intermediate transfer member is reduced. This makes it difficult to accurately detect the position of the image pattern. Thereby, the image forming apparatus finds it difficult to accurately correct the color misregistration.
To suppress influence of the reduction of the reflectance of the intermediate transfer member, Japanese Patent Application Laid-open No. 2007-148080 is proposed. The image forming apparatus described in the Japanese Patent Application Laid-open No. 2007-148080 varies the threshold such that the threshold for converting the analog signal which is output from the optical sensor into the digital signal becomes a value which is lower than a minimum value of the analog signal in accordance with the reflected light amount of the intermediate transfer member.
In manufacturing the intermediate transfer member, an area of locally different reflectance may be generated. Thereby, when performing the color misregistration correction using the intermediate transfer member which is just after the start of use, it is necessary to set the threshold taking into consideration of the local variation of reflectance. However, the image forming apparatus described in the Japanese Patent Application Laid-open No. 2007-148080 determines the threshold based on the same calculation method, which might result in wrongly detecting the area of locally different reflectance as the image pattern. Thereby, it is impossible to accurately detect the position of the image pattern for color misregistration correction, which may decrease the accuracy of the color misregistration correction control. So, the purpose of the present disclosure is to suppress the wrong detection of the image pattern.